This specification refers to several published articles. For convenience, some of these articles are cited in full in a numbered list at the end of the description and cited by number in the specification itself. The contents of these articles, and the various patent documents also referred to hereafter, are incorporated herein by reference and the reader is directed to them for reference.
A Distributed Feedback laser (DFB) differs from an external cavity laser as the feedback mechanism is continuously provided along the gain medium via the monolithic integration of a Bragg structure and a waveguide in the gain medium. Some DFB lasers use surface plasmons as the guiding mechanism and/or a grating to provide the feedback. Tredicucci et al., in European Patent Application EP 1133035 A2 and in “Single-mode surface-plasmon laser”, Applied Physics Letters, vol.76, no. 16, p.2164, 2000, disclose a DFB laser based on a quantum-cascade architecture that uses a metal grating to provide the feedback based on surface-plasmons excited at a single metal-semiconductor interface. The grating is included within the semiconductor laser structure to enhance mode selection, that is, create single mode lasing through Bragg reflection. The grating comprises deposited strips of titanium covered by a thick evaporated layer of gold to create a metal-grating structure with alternating stripes of pure Au and Ti/Au defining the refractive index perturbation.
A monolithic Distributed Bragg Reflector (DBR) laser holds similarities to external-cavity lasers. A monolithic DBR laser is a two mirror laser cavity with, for example, one mirror being a cleaved facet and the other a Bragg reflector waveguide containing a corrugated grating, as described by Coldren in “Monolithic Tunable Diode Laser”, IEEE Journal on Selected Topics in Quantum Electronics, vol.6, no.6, p.988, 2000, and in U.S. Pat. No. 4,896,325. This grating is a narrowband wavelength selective mirror that selects a single longitudinal mode of operation. The highly reflective back facet usually has a reflectivity greater than 90%. A photo-diode can also be used to monitor the output power of the laser through this back facet. A wavelength tunable laser can be obtained from a DBR by injecting a current into the Bragg reflector which perturbs its refractive index and sweeps its center wavelength over a certain range.
External-cavity lasers and tunable external-cavity lasers are well known in the art. Typically, an external-cavity laser (ECL) is composed of an optical gain medium (a laser diode with an antireflection coating on one of two facets) and an external optical system which may include wavelength selective components, coupling optics, and other elements, to establish a lasing cavity thus providing feedback to the gain medium, An external-cavity laser provides many advantages over other integrated laser diode devices such as a Distributed Feedback laser (DFB) or a Distributed Bragg Reflector laser (DBR) or a Fabry-Perot laser. Advantages include extra control in the selection of the optical parameters of the cavity, and narrow line widths. ECLs are also easily pumped, reliable, provide dynamic external control of the selection and tunability of the emission wavelength and provide high efficiency. ECLs are also known to have greater spectral purity and wavelength stability.
Several external-cavity laser architectures can be found in the art. One example of an ECL architecture is the well-known Littrow-grating cavity laser, an example of which, disclosed by Zorabedian and Trutna in “Alignment Stabilized Grating Tuned External Cavity Semiconductor Laser”, Optics Letters, vol.15, p.483, 1990, combines a gain chip, a collimating lens and a rotatable diffraction grating providing frequency selective feedback. U.S. Pat. No. 5,392,308 discloses a Littrow external cavity laser similar to the above where a tapered stripe gain chip is used.
Fleming and Mooradian in “Spectral Characteristics of External-Cavity Controlled Semiconductor Laser”, IEEE Journal of Quantum Electronics, QE-17(1), p.44–59, January 1981, describe a grating-tuned double cavity laser. This ECL combines an optical amplifier with antireflection coatings on both facets to couple to two external cavities. One external cavity comprises a collimating lens and a diffraction grating for frequency selection and the other external cavity comprises a collimating lens and a partially reflective mirror for output coupling.
A fiber Bragg laser is another well known ECL. U.S. patent application Ser. No.2002/0015433 A1 by Zimmermann discloses a standard fiber Bragg laser comprising a gain chip with a highly reflecting facet that is coupled to a lensed fiber which contains a fiber Bragg grating. It is known that the lensed fiber can be replaced by a tapered fiber or a cleaved fiber coupled to a free space lens. Zimmermann also discloses a fiber Bragg external cavity laser which uses a distributed Bragg reflector (DBR) defined in the gain medium and a super-structure fiber Bragg grating (SSG) in the external cavity. A second disclosed laser uses a super-structure distributed Bragg reflector defined in the gain medium and a super-structure Bragg grating defined in the fibre as the reflective ends of the cavity, with the output being taken from the fibre grating.
Such previously-known external cavity lasers suffer from several limitations. These limitations may include, among others, high fabrication costs, large device sizes, weak polarization extinction ratios and elaborate wavelength tuning schemes.
It is often desirable for an external cavity laser to supply linearly-polarized light. Examples of external cavity lasers which include a polarizer and supply polarized light are discussed below.
Thus, in U.S. Pat. No. 5,734,667, Esman et al. disclose means for controlling the state of polarization in the resonant cavity of an external cavity doped fibre laser. The means disclosed comprise a number of optical elements, including reflectors, Faraday rotators and a polarizer, assembled using fibre sections or optical lenses.
In U.S. Pat. No. 4,479,224, Rediker discloses an external cavity laser comprising an array of semiconductor optical gain elements and reflectors. One of the reflectors may be a grating to enable wavelength selection. Means for ensuring that a known polarization state is emitted from the laser is also disclosed, such means implemented as a polarizer inserted into the optical path within the external cavity.
In U.S. Pat. Nos. 6,381,259 and 6,181,728, Cordingley et al. disclose an apparatus for modifying the state of polarization emitted by a laser cutting device. The apparatus ensures a linearly or elliptically polarized emission by placing in the optical output path of the laser a liquid crystal polarization modification element and a polarizer.
In U.S. Pat. No. 5,218,610, Dixon discloses an external cavity laser having a cavity defined by end mirrors with a polarizer and a voltage controlled variable liquid crystal waveplate in the light path between the end mirrors. The gain medium “sees” a reflectivity that is wavelength dependent and controllable by varying the voltage applied to the waveplate.
Disadvantages of such arrangements are increased losses and complicated assembly, since the polarizer and additional components need to be aligned along the optical path within the cavity, or along the optical path outside of the cavity.
In U.S. Pat. No. 4,009,933, Firester discloses a polarization selective mirror which reflects broadband TE and TM polarized light, but with only a 7% difference in the reflectivity of these states. According to Firester, such a device could be used as a reflector in the external cavity to help select a lasing polarization state. However, the difference in the reflectivity of this device is low and the reflector is not wavelength selective.
Bischel et. al disclose in U.S. Pat. Nos. 5,499,256 and 5,513,196 an external cavity laser that employs a polarization converter in the external cavity to achieve lasing in a single polarization. When discussing the prior art, Bischel et al. describe problems associated with a known external cavity laser by Heismann et al. which uses a waveguide that supports both TM and TE polarization modes and uses a thin film polarizer to limit the polarization to the TE mode prior to the output reflecting mirror. Bischel et al. point out that fabrication of Heismann et al.'s device is complicated because four transverse degrees of freedom must be controlled mechanically during the alignment of the laser chip and the polarizer. In addition, Bischel et al. point out that the thin film polarizer is a lossy element. Bischel et al. sought to overcome these alignment problems by means of a planar construction in which a TE-polarized diode laser is butt-coupled to a first waveguide which converts TE polarization to TM polarization (and vice-versa), the output of which is coupled to a second waveguide which supports only TM polarization. A limitation of such an ECL is that the TE-TM mode converter has an optical bandwidth (FWHM) about 1 nm; thus the line width of the emission is not expected to be very narrow.
Generally, all of these polarized-output external cavity lasers suffer from one or more of the following limitations: high fabrication costs, large device size, weak polarization extinction ratio, wide line width and elaborate wavelength tuning and stabilization schemes.